DE 103 06 857 A1 discloses a method for determining a fuel content in the oil for the lubrication of a fuel pump, wherein the temperature of the lubricant or a component temperature of the fuel pump or of the internal combustion engine is determined. This however already assumes elevated friction in order that the friction can lead to an excessive increase in temperature, which is to be ascribed as a reaction to the reduced viscosity of the oil. A disadvantageous consequence of the viscosity reduction is thus provoked and awaited, and then detected. This is however considered to be disadvantageous for the oil circuit.